The present invention relates to casters and in particular to casters which include wheels that are rotatable about a tilted or inclined axle. Casters of this type are used to support articles of furniture such as chairs for rolling over rugs or other flooring surfaces.
It has been found desirable to provide such a caster with one frusto-conical shaped wheel rotatably attached to each respective axle. Such a caster provides a low profile and a wide footprint such that the depression of the wheels into the surface will be minimized. These and other advantages of having a single frusto-conical shaped wheel rotatably attached to a single tilted axle are discussed in Rivkin et al. U.S. Pat. No. 4,752,986. The wheels used in Rivkin et al. include a frusto-conical shaped floor engaging surface which extends between an inner rim, having a relatively large diameter, and an outer rim having a relatively small diameter.
While such a design provides the desired wide footprint characteristic, the frusto-conical surface of the wheels shown in the Rivkin et al. patent will slip or drag on the floor surface as the caster is moved in a linear direction. The slipping or dragging of the wheels in the Rivkin et al. caster is due to the large difference in the diameters of the inner and outer rim of each wheel which results in a large difference in the length of the respective circumferences of the inner rim and of the outer rim. For example, when the Rivkin et al. caster is moved in a linear direction a distance equal to the circumference the larger inner rim, the inner rim will have made one complete revolution. Since the inner rim and outer rim are connected, the outer rim will also have made one complete revolution. However, since the outer rim has a smaller circumference than the inner rim, one revolution of the outer rim will not cover the same distance as one revolution of the inner rim. Therefore the outer rim must be partially rotated on, and partially dragged along, the floor surface. In the converse situation, if the caster moves a distance equal to the circumference of the outer rim and the outer rim has made one revolution, the inner rim must partially rotate on the surface and must partially slip or slide along the floor surface to keep pace with the outer rim. Such dragging or slipping by the wheels causes marring of the floor surface which is aesthetically undesirable and which can also damage the surface. The dragging and slipping of the wheel also impedes the overall ease of movement of the caster.